In computing systems, such as desktop computers, portable computers, personal digital assistants (PDAs), servers, and others, storage devices are used to store data and program instructions. One type of storage device is a disk-based device, such as a magnetic disk drive (e.g., a floppy disk drive or hard disk drive) and an optical disk drive (e.g., a CD or DVD drive). Disk-based storage devices have a rotating storage medium with a relatively large storage capacity. However, disk-based storage devices offer relatively slow read-write speeds when compared to operating speeds of other components of a computing system, such as microprocessors and other semiconductor devices.
Another type of storage device is a solid state memory device, such as a dynamic random access memory (DRAM), static random access memory (SRAM), flash memory, and electrically erasable and programmable read-only memory (EEPROM). Although solid state memory devices offer relatively high read-write speeds, usually on the order of nanoseconds, they have relatively limited storage capacities.
With improvements in nanotechnology (technology involving microscopic moving parts), other types of storage devices are being developed. One such storage device is based on atomic force microscopy (AFM), in which one or more microscopic scanning probes are used to read and write to a storage medium. Typically, a scanning probe has a tip that is contacted to a surface of the storage medium. Storage of data in the storage medium is based on perturbations created by the tip of the probe in the surface of the storage medium. In one implementation, a perturbation is a dent in the storage medium surface, with a dent representing a logical “1,” and the lack of a dent representing a logical “0.” Other types of perturbations that can be created in the surface of the storage medium include creating or altering the topographic features or composition of the storage medium, altering the crystalline phase of the medium, filling or emptying existing electronic states of the medium, creating or altering domain structures or polarization states in the medium, creating or altering chemical bonds in the medium, employing the tunneling effects to move and remove atoms or charge to or from the medium, or storing/removing charge from a particular region.
Traditionally, for a probe to read data bits from a storage medium in which dents have been formed, the probe is heated to a predetermined temperature, such as 400° C. When a scanning tip of the probe encounters and enters a dent, the tip transfers heat to the storage medium, which causes the temperature of the probe tip to fall, which in turn causes electrical resistance of the tip to fall. The fall in resistance, which is a relatively tiny amount, is detected by detection circuitry to determine the state of the data bit. Another technique for detecting the state of a data bit involves the use of a piezoresistive element in the probe. When the probe tip encounters a dent, the cantilever of the probe deflects, which causes the resistance of the piezoresistive element to change. The change in resistance is measured by detection circuitry. However, with the above techniques, the variation in resistance due to the presence and absence of a dent is relatively small. As a result, reliable detection of data bits may not always be possible due to the presence of noise and other factors. Also, the speed at which read operations can be performed is also reduced due to weak read signals in such probe-based storage devices.